Lucas/Peach/Zelda/Shiek are my problem characters atm.
Falco too.
Ugh i need to get better.
Falco, I have a lot of exp vs this character.
Spin a lot, My 2 least favorite stages vs Falco are FoD and Yoshis story from melee. The platforms let him get away from your spinning so he can come from above and combo you. So I recommend banning these stages vs him, and same with Fox and Wario.
When Falco is recovering, your homing attack (neutral B) ***** his recovery, but you have to be quick enough so that his firebird is still charging when you hit it. If the Firebird is moving you will trade with it which might still kill him if hes far enough from the ledge. (also he might try to illusion but if you are close enough, neutral B will auto lock on but it has to hit him before the illusion comes out though. Falco's usually Illusion if they have their DJ, so the moment they DJ I use neutral B because I know an illusion is probably coming.)
While onstage, try to side B into him, Sonic's new side B gets Fast-fallers/heavy characters basically be stuck inside your spin which lets you bring them offstage if you are close enough to the ledge where you will go off it. Or, right before you go offstage with them, press A to stick your foot out to kick them giving some knockback. Once, you send them offstage you usually want to edgeguard them with neutral B since it will lock onto him.
(P.S. whenever you side B, make sure you don't hold side B at all or else you will do that little jump, similiar to Sonic's side B in Brawl.) If you quickly tap Side B (the speed you want to input it at would be the same speed you would use to short hop) you will stay on the ground the whole time you are spinning. You almost never should use the little jump on the side B. Always just have Sonic stay on the ground throughout the spinning animation.
Also, always Up-throw spacies, if they DI so they are directly above you, you should probably up tilt or re-grab them. If they DI in front of you, Side B them before they touch the ground so they can't tech.
Hope this wasn't confusing lol.
Only, character specific stuff I remember about sheik is that her needles clank with your spin dash (at least in 2.1)